The invention relates to processes for control of crystallization of inorganics, such as Na, Li, K, Ca and Mg bicarbonates, phosphates, borates, hydroxides, nitrates and sulfates from aqueous solutions containing the respective ions thereof. More particularly, the invention relates to the use of lecithin as a generally regarded as safe (GRAS) food grade and Kosher crystal growth promoter and crystal size classifier (size control agent) for the crystallization of sodium bicarbonate from solution mined Nahcolite pregnant liquor.
Global sodium bicarbonate demand in 1996 is projected at one million tons per year, with U.S. capacity at about two thirds that amount. There are four major processes for production of sodium bicarbonate, all of which involve crystallization from aqueous solution. The most commonly currently used process involves mining impure Trona mineral which is purified by recrystallization from a hot aqueous solution via intermediate steps of production of sodium carbonate or sesquicarbonate and recarbonation with CO2. A second method involves carbonation of brines rich in sodium, borate, carbonate and bicarbonate ions, such as lake brines, e.g. brines of Owens Lake, Lake Natrona and from Searles Lake, Calif., and Lake Magabi, Tanzania. Still another method involves the production of sodium bicarbonate in the ammonia soda process. Finally, one of the lowest cost processes is solution mining Nahcolite mineral from the evaporate deposits of the Green River Formation in the Piceance Creek Basin, Colo. according to the Nahcolite Solution Mining Process of the: Rosar and Day. U.S. Pat. No. 4,815,790. The pregnant liquor is pumped above ground where it is crystallized at atmospheric pressure and ambient temperature in a series of crystallizers staged in parallel or series.
Crystal nucleation and growth rate from a solution is expressed in terms of crystallization kinetics. Crystal habit is the shape which results from the different rates of growth of the various crystal faces. Both crystallization kinetics and crystal habit influence production costs, product purity, caking, bulk density, dusting, flowability and the like. Both brine concentration, natural and/or induced solution impurities even in low concentrations, impact crystallization kinetics and crystal habit in commercial crystallization operations. Ordinarily, the nature and extent of the impact of such impurities is both adverse and unpredictable.
Impurities in brine or solution mined Nahcolite pregnant liquor change from time to time. These impurities are both inorganic and organic. For example, in Nahcolite solution mining, the Nahcolite beds are typically some 2,000 feet below the surface. Nahcolite beds typically include inter-bedded stringers, lenses or rosettes of kerogen-containing shale, salt or Dawsonite mineralization, among others. Thus, commercial operations often experience xe2x80x9cdriftxe2x80x9d in which, due to subtle variations in impurity content over time even while maintaining the same mining and crystallizer conditions of temperature, agitation, heat exchange and throughput rate, there can be vast differences in the end product. For example, there can be relatively wide swings in the quantities of xe2x80x9coversizedxe2x80x9d or xe2x80x9cundersizedxe2x80x9d product, and in the bulk density, caking and friability of the product. That is, the percentage of crystals which are too large and percentage of very fine crystals (called xe2x80x9cfinesxe2x80x9d), may vary over time, in some instances as quickly as within a few days. Where the product crystals are too large, the product becomes unsuitable or uneconomic to use, particularly in food products, or as an SOx sorbent in pollution control processes, one of the significant uses of sodium bicarbonate. Likewise where the product is too fine, it is difficult to dewater during processing, increases process energy costs, and the resulting product cakes easily, creates dust when handled and is difficult for the customers to use.
As pointed out in the Bauer et al. U.S. Pat. No. 3,072,466, bicarbonate crystals obtained by various commercial crystallization processes in many cases are of inferior quality considering such factors as crystal shape, purity, settling rate, size, uniformity, dewaterability, bulk density and resistance to breakage during handling. The Bauer et al. patent is directed to the use of anionic-active surfactants of the organic sulfate or sulfonate-type derivatives, and most preferred are the alkyl benzene or alkyl naphthalene sulfonates. The Bauer et al. patent also teaches that cationic and nonionic surfactants are xe2x80x9ctotally ineffective as additives in improving the crystallization of sodium bicarbonate.xe2x80x9d It states that xe2x80x9cvarious theories have been considered in an effort to explain the clearly established, unique effectiveness of the anionic-active surfactantsxe2x80x9d and xe2x80x9cthe complete lack of effectiveness of the cationic and nonionic classes,xe2x80x9d but those theories xe2x80x9chave all failed to fully explainxe2x80x9d the xe2x80x9cunexpected resultxe2x80x9d of anionic surfactant activity.
Crystal growth modifiers can impart positive and/or negative influences, with the goal being to enhance the positive and reduce or eliminate the negative. In inorganic solution crystallization, the manner in which modifiers function on the molecular level is unpredictable and speculative.
There is another factor involving such modifiers. Sodium bicarbonate is used in many food products and processes. Thus, crystallization agents such as the alkyl benzene or naphthalene sulfonates do not have GRAS classification, and at best may only be sparingly used in food grade sodium bicarbonate production. The field is replete with attempts to use commercial dishwasher detergents such as DBSA, Petro AG (DeSoto Chemical Company) with diesel fuel, kerosene or styrene. All of these have serious food grade and GRAS non-approval issues, and are not very efficient modifiers. Other additives have been tried, such as hexametaphosphate, but such additives result in an extremely high loss in yield and extremely dendritic crystal habit which makes them unsuitable for use.
It is among the objects and advantages of the invention to provide a GRAS crystal growth modifier for crystallization of inorganics from aqueous solutions containing Na, Li, K, Ca and Mg cations, and carbonate, bicarbonate, borate, hydroxide, nitrate and sulfate anions, which modifier is a selective crystal size classifier and limiter. It is another object and advantage of the invention to employ lecithin full strength, or in solutions, suspensions, mixtures and emulsions in minor (parts per million) quantities, as an extremely good crystal growth promoter, size classifier, dewatering agent and supersaturation reducing agent. It is still another object and advantage of the invention to provide a process employing adding lecithin in parts per million quantities to aqueous inorganic solutions, for example, of sodium bicarbonate from a variety of sources, including Nahcolite solution mining and brine mining pregnant liquor, as an improved crystal growth promoter and which reduces both heat exchanger and crystallizer scaling. It is another object and advantage of the invention to provide a method for introduction of lecithin in controllable parts per million quantity to aqueous solutions of inorganic ions in a simple, reproducible and effective manner without the need for additional emulsification, detergent, saponification or Ph control agents. These and other objects and advantages are evident from the description of the invention herein.
The invention comprises the use of parts per million quantities of lecithin, particularly lecithin extracted from soybean oil, in the range of from about 2 to about 200 ppm, and more particularly in the range of from about 5 to about 60 ppm as a crystal growth promoter and size classifier for crystallization of inorganics from solutions containing the above-listed cations and anions. By xe2x80x9csize classifierxe2x80x9d is meant the property of narrowing the range of crystal sizes, especially to within the useful product range of xe2x88x92425 xcexcm+45 xcexcm, i.e., xe2x88x9235 mesh+325 mesh (Tyler mesh).
The lecithin may be added to the inorganic solution in one or more crystallizer tank(s), flow lines, and/or pregnant liquor tanks. It may be added directly in non-emulsified or non-solvated, unmixed, as received, full strength form in commercial operations and this form of addition is best accomplished by high shear mixing. The beneficial effects are slow to develop with slow mixing. To achieve best results, the full strength lecithin is best added upstream of high speed rotating pump impellers. Crystallization of sodium bicarbonate is discussed in detail herein by way of example and not by way of limitation of the principles of the invention.
An alternative and equally preferred method is to premix the lecithin with pregnant or barren liquor in the range of from room temperature to 250xc2x0 F. to form a solution on the order of from 200 to 20,000 ppm, and more preferably from 200 to 10,000 ppm, and introduce this mixture into one or more crystallizer tanks, pregnant liquor tanks or other suitable injection point in the process. Premixing with hot or warm pregnant liquor is preferred so there is less process dilution, but if long shelf life at room temperature is required, barren or at best near saturated (with respect to CO3xe2x88x92/HCO3xe2x88x92 ions) is preferred as less likely to crystallize out pending use. The lecithin pregnant or barren liquor mixture is termed xe2x80x9cthe additive mixture,xe2x80x9d but it should be understood that term includes true or partial emulsions, suspensions, or solutions of lecithin in the aqueous carrier or other solvent. The pH can range from about 8 to 12 and is preferred to be in the range of from about 8 to about 10, and carbonate and/or bicarbonate ions are preferably present in the additive mixture. When the term xe2x80x9clecithinxe2x80x9d is used herein, it should be understood to include both full strength lecithin per se and the additive mixture, or equivalent mixtures, emulsions, suspensions or solutions in an aqueous carrier or other solvent.
It should be understood that by using full strength lecithin or the additive mixture as a crystal growth promoter, it is possible to adjust the quantity of use thereof to produce large crystals on the order of the size of beach sand in the range of {fraction (1/64)} inch to xe2x85x9 inch. These crystals are extremely useful as a sandblast agent which, unlike sand does not raise serious health (silicosis) and environmental issues. Accordingly, while the above-mentioned ranges for the use of lecithin are preferred for producing food grade bicarb product crystals in the preferred range of xe2x88x92425 xcexcm +45 xcexcm in size, there may be other uses for larger or smaller products. However, the inventor has found, for example, the use of from about 5 to about 30 ppm of lecithin, and preferably 10xc2x1ppm, in a commercial plant demonstration not only eliminates the oversize (crystals of size not passing a 425 xcexcm opening screen) but also the undersized, (that is, the quantity of crystals passing through a screen having 45 xcexcm openings) are reduced by some 33%.
It is important to note that in bench scale testing the amount of lecithin, expressed in ppm, is some 3-10 times greater than that required in actual plant (commercial) operations. For example, optimum in bench scale is in the 20-60 ppm range, while the range of 5-20 ppm is optimum for the xe2x88x92425 xcexcm+45 xcexcm product in current commercial operation, and 5-15 ppm is the most preferred operating range in the plant.